Fire control system for multi-zone buildings

ABSTRACT

A method of controlling smoke, ventilation and fire flash point temperature in a multi-zone building served by a central ventilation system uses a divided common return air duct having separate branch damper connections for each zone for directing all return air from a fire zone into one section of the common return air duct for complete exhaust and for directing all return air from a non-fire zone into the other section of the common return air duct. The method also provides positive pressurization of the non-fire ventilation zones relative to the fire ventilation zones and maximum fresh air to all zones.

Unite States Patent Miller FIRE CONTROL SYSTEM FOR MULTl-ZONE BUILDINGSEdward J. Miller, 6702 Church St., Morton Grove, 111. 60053 Filed: Aug.21, 1974 Appl. No.: 499,129

Inventor:

U.S. Cl 98/33 R; 165/16; 169/61; 236/49 Int. Cl. A62b 15/00; A620 3/14Field of Search 98/33 R, 43; 236/49; 169/60, 61; 165/16 References CitedUNITED STATES PATENTS 3/1971 McLeod 236/49 C 6/1973 Knapp 236/49 X [451May 20, 1975 Primary Examiner-William E. Wayner Attorney, Agent, orFirm-J. Patrick Cagney [57] ABSTRACT A method of controlling smoke,ventilation and fire flash point temperature in a multi-zone buildingserved by a central ventilation system uses a divided common return airduct having separate branch damper connections for each zone fordirecting all return air from a fire zone into one section of the commonreturn air duct for complete exhaust and for directing all return airfrom a non-fire zone into the other section of the common return airduct.

The method also provides positive pressurization of the non-fireventilationzones relative to the fire ventilation zones and maximumfresh air to all zones.

14 Claims, 8 Drawing Figures PATENTED MAY 2 0 I975 SHEET 10? 4 PEG. 2

PATENTEWZO'WS $884,133

SHEET 20F 4 PATENMBHAYZUIUYS 3,884,133

SHEET 30F 4 FIG. 5

FIRE CONTROL SYSTEM FOR MULTI-ZONE BUILDINGS BACKGROUND OF THE INVENTIONThis invention relates to fire control systems for multi-zonedbuildings, such as high rise office or apartment buildings, that areserved by a central ventilation system.

It is generally recognized that effective control of smoke, ventilationand fire flash point temperature requires a system capable ofproviding 1. maximum pressurization of the non-fire zones together withgood exhaust of ambient smoke;

2. maximum fire zone ventilation to prevent flashover temperatures; and

3. maximum fire side exhaust for smoke removal.

There has been a continuing need for a system capable of satisfyingthese criteria. Even though the importance of adequate fire control forhigh rise buildings is recognized by all, no truly. effective solutionhas previously been developed,

SUMMARY OF THE INVENTION The present invention provides a flexiblesystem for controlling smoke, ventilation and fire flash pointtemperature that is simplified and economical and that satisfies theaforementioned criteria by providing 1. 100 percent air to the fire andnon-fire zones;

2. reduced return air from the non-fire zones;

3. a multifold increase in return air from the fire area to exhaustsmoke and heat and cause a larger percentage of the total fresh air tobe delivered to the fire zone than under normal conditions for fireventilation; and

4. maximum fresh air to both fire and non-fire zones.

More particularly, the invention provides a method of fire control andventilation for multi-zone buildings comprising drawing fresh air intothe building to form a main supply stream of supply air; subdividing themain stream into branch streams and supplying each branch stream into aseparate zone; drawing air from each zone to form a pair of commonreturn air streams; normally exhausting at least some of the air fromthe return air streams while recycling the remainder into the mainsupply stream; and responding to fire conditions occurring in any one ofthe zones to depressurize any fire zone by directing all the air drawnfrom such zone into one of the return air streams while exhausting allof said one return air stream to pressurize the nonfire zones bydirecting all the air drawn from the nonfire zones into the other returnair stream.

AS applied to a typical multi-zone building arrangement that has aventilation system including a central station serving a plurality ofventilation zones, common supply air duct means connecting theventilation zones to the central station, common return air duct meansconnecting the ventilation zones to the central station, each zonehaving branch supply duct means connected to the common supply air ductmeans and branch return duct means connected to the common return airduct means, and the central station including central supply fan means,central exhaust return fan means and main damper means for controllingflow of fresh air into the common supply air duct means and recycling ofreturn air from the common return air duct means to the common supplyair duct means, the improvement of this invention comprises partitionmeans for dividing the common return air duct means lengthwise into amain return air duct section and an auxiliary return air duct section,separate branch damper means for each zone for controlling communicationof the branch return duct means of such zone with each of the return airduct sections, standby means for effecting isolation and exhaust of allreturn air from the auxiliary return air duct section, and control meansincluding separate detection means for each zone operable upon theoccurrence of fire conditions therein for actuating the standby meansand the corresponding one of said separate damper means for directingall of the return air from any fire zone to exhaust through theauxiliary return air duct section, and for directing all of the returnair from any non-fire zone into the main return air duct section.

Additional features of the presently preferred embodiments include: theprovision of a standby exhaust fan for connection to the auxiliaryreturn air duct section in the fire mode to exhaust all return air fromfire zones; an automatically responsive control system for sequencingthe positioning of all necessary dampers both in the normal mode and inthe fire mode; the provision of static pressure means for controllingthe capacity of the standby means for effecting a pressure reduction inthe fire zones; the provision of facilities to enable supply of percentfresh air to all ventilation zones during the fire mode; and theprovision of sprinklers for controlling return air temperature.

Other features and advantages of the invention will be apparent from thefollowing description and claims and are illustrated in the accompanyingdrawings which show structure embodying preferred features of thepresent invention and the principles thereof, and what is now consideredto be the best mode in which to apply these principles.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic vertical sectionalview showing a multi-zone high rise building equipped with a centralventilation system incorporating the fire control system of thisinvention;

FIG. 2 is a fragmentary sectional plan view taken as indicated on theline 2-2 of FIG. 1;

FIG. 3 is a diagrammatic plan view of the central fan room of theventilation system;

FIG. 4 is a diagrammatic view similar to FIG. I and showing theinvention as applied to a ventilation system that uses free return flowfrom a ceiling plenum arrangement on each floor;

FIG. 5 is a fragmentary plan view taken as indicated on the line 5-5 ofFIG. 4;

FIG. 6 is a plan view of a horizontal type multizone ventilation system;

FIG. 7 is a view similar to FIG. I and showing a modified fan system toillustrate other adaptations of the system; and

FIG. 8 is a schematic control circuit diagram for the ventilationsystems disclosed herein.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to the drawings, animproved ventilation system to provide for smoke, ventilation andtemperature control in fire management is illustrated in connection witha multi-story building in FIG. 1 wherein 1O designates the floor linefor a central fan room at the top of the building, and 9, 8, and 7designate the floor lines for lower floors of the building, each ofwhich constitutes a separate ventilation zone, in the case of theillustrated embodiment, with all of the ventilation zones being servedby a common ventilation system.

Such a common ventilation system typically includes a central station orfan room 11, a common supply air duct 12 connecting the central stationwith the ventilation zones and a common return air duct 13 connectingthe ventilation zones to the central station, with each ventilation zonehaving a branch supply air duct 14 connected to the common supply airduct 12 and a branch return air duct 15 connected to the common returnair duct 13.

The central station typically includes a central supply fan 16 mountedin the top of the common supply air duct 12, a central exhaust/returnfan 17 mounted in the top of the common return air duct 13 and a set ofmain dampers for controlling the flow of fresh air into the commonsupply air duct and for controlling recycling of return air from thecommon return air duct 13 to the common supply air duct 12. As shown,the main dampers includes a fresh air supply damper 18 mounted in afresh air supply duct 19 that feeds the supply fan, a main exhaustdamper 20 mounted in a main exhaust duct 21 that exits from the returnair duct and a return air shut-off damper 22 mounted between the supplyfan 16 and the exhaust/return fan 17.

In accordance with this invention improvements are incorporated in thecentral ventilation system to provide for more effective fire managementby isolating return air from any fire zone from return air from anynon-fire zone and exhausting all of the return air from the fire zone sothat the non-fire zones are not affected by any smoke or other fireconditions. In addition the non-fire zones are preferably operated atelevated pressure and the fire zones are maintained at reduced pressureto assist in isolating the fire zones.

In the illustrated embodiment of FIGS. 1 to 3 the ventilation system isprovided:

with partition means 23 for dividing the common return air duct 13lengthwise into a main return air duct section 13M and an auxiliaryreturn air duct section 13A;

with separate branch damper means 24 for controlling communication ofthe branch return air duct 15 of each zone with each of the return airduct sections 13M, 13A;

with stand-by means 25 for effecting isolation and exhaust of all returnair from the auxiliary return air duct section in the fire mode;

and with control means 26 which may include connec tions to a centralcontrol panel 27 at any suitable location in the building and which mayinclude separate detection means 28 for each zone, the detection meansbeing operable upon the occurrence of fire conditions for actuating thestandby means 25 and the separate branch damper means 24 for directingall the return air from a fire zone to exhaust through the auxiliaryduct section 13A and for directing all the return air from a non-firezone into the main return air duct section 13M.

Stand-by means 25 as shown here includes the standby exhaust fan 29located in the central fan room and having a backdraft damper 29D forcontrolling flow from the auxiliary duct section 13A to exterioratmosphere and also includes an auxiliary damper 30 located in the topof the auxiliary duct section 13A for isolating the same from both themain supply air duct 12 and from the main return air duct section in thefire mode, thereby directing all the return air from the fire zonesthrough the stand-by exhaust fan 29.

As shown in FIG. 8 the central control panel 27 is individuallyconnected to each of the detectors 28 and to the branch dampers 24 forautomatically controlling the same and is also connected to the maindampers for automatically controlling these in the fire mode, as well asfor controlling the stand-by exhaust fan 29 and the auxiliary damper 30.

It should be noted that while the partition 23 is shown centrally of thecommon return air duct 13 it may be located to determine a differentrelative size relationship between the main section and the auxiliarysection. Moreover, these sections may be separate ducts rather than adual duct as shown. It is contemplated that numerous other variationsmay be made in adapting the system of this invention to the requirementsof a particular building or of the local fire codes.

It should also be noted that the branch dampers 24 consist of dualsections 24M, 24A located in the connection between each branch returnair duct and the main and auxiliary return air duct sections 13M, 13Arespectively.

During normal operating conditions, both dual damper sections 24M, 24Aare open but upon activation of the detector 28 on a fire floor, themain damper section 24M is closed and the auxiliary damper section 24Ais open. Simultaneously, on each of the non-fire floors the main dampersection 24M is closed and the auxiliary damper section 24A is open,under the control of the corresponding detectors. Subsequentdeactivation of a detector on any floor shifts the corresponding branchdamper to the same position then occupied by the branch dampers on thenon-fire floors until the last fire floor detector becomes deactivatedwhen the entire system would revert to normal.

The detectors may be of any suitable type selected to respond to smokeor heat or ionization products. Depending on requirements of the localfire codes, the detectors are electrically connected to the centralcontrol panel so that when activated any detector will transmit acontrol pulse to the central control panel which is programmed to supplyactuating signals to convert the entire system from normal mode to firemode.

NORMAL MODE In normal operation fresh air is drawn into the buildingthrough duct 19 to form a main supply stream in the common main duct 12;the main supply stream is subdivided into branch streams in the branchsupply ducts 14 that feed each of the separate ventilation zones; returnair is drawn from the ventilation zones to form a pair of common returnair streams in the ducts 13M, 13A; and a portion of the return airstreams is exhausted through duct 21 while the remainder is recycledinto the main supply stream in duct 12. it is presently preferred thatthe ventilation system operate at all times to provide the minimum airmovement required to faciliate fire detection by the detector 28 in anyzone.

FIRE MODE When fire conditions are sensed by a detector 28 in any one ofthe ventilation zones, an electric impulse actuates the control panel 27to reposition the components for directing all the air from any firezone into the auxiliary return air duct 13A and through the standby fan29 to the exterior while directing all the air from any non-fire zoneinto the main return air duct 13M for partial or complete exhaust asselected. During fire mode, the supply air continues to all zones andmay consist of 100% fresh air or ofa mixture of fresh air and return airrecycled from the non-fire zones.

More specifically, for the embodiment of FIGS. 1 to 3, the activation ofany detector 28 causes the control panel to isolate the auxiliary duct13A by closing damper 30, activating the stand-by fan 29 and opening itsback damper 29D, and for each fire zone closing its branch damper 24Mand for each non-fire zone closing its branch damper 24A.

When any fire zone is cleared as sensed by its detector 28 while otherfire zones still exist, the cleared zone has its branch dampersautomatically reset to open damper 24M and to close damper 24A.

If 100 percent fresh air is to be supplied during fire mode, the recycledamper 22 is also closed and a reheat coil 31 located in the main supplyduct is activated as required.

A static pressure control 32 is connected to control the capacity of thestandby fan 29 to provide a variable volume capability for effecting thedesired pressure reduction in the fire zone. Typically, the standby fan29 would be provided with adjustable vanes controlled by the staticpressure device to achieve this function.

Finally, a variable volume sprinkler 33 is located in the branch returnduct of each zone and includes a sprinkler head and a temperaturedetector located at each floor line to control exhaust temperature fromeach fire zone.

ADDITIONAL EMBODIMENTS A modified embodiment ofa typical floor is shownin FIGS. 4 and 5 wherein the return air is drawn from a free return suchas a plenum ceiling into the main and auxiliary return air duct section13M, 13A respectively. The main damper section 24M is omitted and theauxiliary damper section 24A is open during normal operations. Operationof any separate detection means 28 in any zone upon occurrence of fireconditions therein causes an electric impulse to activate the controlpanel 27 to close the auxiliary damper section 24A in each of thenon-fire zones and reposition the components for directing all the airfrom any fire zone into the auxiliary return air duct 13A and throughthe standby exhaust fan means 29 to the exterior.

Another embodiment as shown in FIG. 6 illustrates the partition means 23extended into the horizontal branch return air duct serving each zone toachieve horizontal sub-zoning in addition to the vertical zoningillustrated in FIG. 1. The separate branch damper means 24 forcontrolling communication of the branch return air duct 15 of each zonewith each of the return air duct section 13M, 13A is relocated as shownto control communication of the branch return air duct 15Z within eachhorizontal sub-zone with each of the branch return duct section 15M,15A. The separate branch damper means 24 is open during normaloperation. Operation of any separate detection means 28 in any sub zoneupon occurrence of fire conditions therein causes an electrical impulseto activate the control panel 27 to close the auxiliary damper section24A in all non-fire zones and sub-zones and close the main dampersection 24M in the fire sub-zone and reposition all components fordirecting all the air from the fire sub-zone into the branch return ductauxiliary section 15A and the auxiliary return air duct 13A and throughthe standby exhaust fan means 29 to the exterior. All other componentswould operate as described for FIG. 1.

Still another embodiment, as shown in FIG. 7 utilizes a system similarto that in FIG. 1 except that the standby exhaust fan means 29 isomitted and the central exhaust/return fan 17 is replaced by twoparallel fans, the non-fire area central exhaust/return fan 17M and thefire area auxiliary exhaust/return fan 17A; the dividing barrier 23 isextended into the main exhaust duct 21 dividing this duct into a generalarea main exhaust duct 21M and a fire area auxiliary exhaust duct 21A,controlled respectively by the general area main exhaust duct damper 20Mand the fire area auxiliary exhaust duct damper 20A; the auxiliarydamper 30 is retitled 30A and moved to a point just after the fire areaauxiliary exhaust/return fan 17A to isolate the auxiliary return airduct section 13A and the fire area auxiliary exhaust duct section 21A,along with the fire area auxiliary exhaust/return fan 17A, from the mainreturn air duct section 13M and the non-fire area central exhaust/returnfan 17M during fire mode; the static pressure control 32 is relocated toa point just before the fire area auxiliary exhaust/return fan 17A.

Normal Operation of FIG. 7 Embodiment The auxiliary damper 30A is wideopen and the gen eral area main exhaust duct damper 20M and the firearea auxiliary exhaust duct damper 20A operate together in conjunctionwith the recycle damper 22 and the fresh air supply duct damper 18 toprovide fresh air as desired to the conditioned area.

Fire Mode Operation of FIG. 7 Embodiment The auxiliary damper 30A closesand the fire area auxiliary exhaust duct damper 20A opens wide and allthe air from the fire zone is exhausted to the exterior atmosphere bythe fire area auxiliary exhaust/return fan 17A. The non-fire areacentral exhaust/return fan 17M continues to deliver return air to thecentral supply fan 16. The general area main exhaust duct damper 20Moperates in conjunction with the recycle damper 22 and the fresh airsupply duct damper 18 to provide the maximum amount of fresh airthroughout the building consistent with exterior temperature and theheating capability of the reheat coil 31 to avoid freeze-ups during coldweather. The relative sizing of the two'parallel exhaust/return fans17M, 17A respectively can be varied according to local codes and designrequirements as to the desired pressure differential between the fireand non-fire zones. Alternately, the non-fire area centralexhaust/return fan 17M could be a two speed fan to handle, at fullspeed, percent return air from the non-fire areas during fire mode, or,at 1% speed, 50 percent of the return air from all areas during normalmode.

Thus, while preferred constructional features of the invention areembodied in the structure illustrated herein, it is to be understoodthat changes and variations may be made by those skilled in the artwithout departing from the spirit and scope of the appended claims.

I claim:

1. In a multi-zone building having a ventilation system that includes acentral station serving a plurality of ventilation zones, common supplyair duct means connecting the central station to the ventilation zones,common return air duct means connecting the ventilation zones to thecentral station, each zone having branch supply duct means connected tothe common supply air duct means and branch return duct means connectedto the common return air duct means, and the central station includingcentral supply fan means, central exhaust/return fan means and maindamper means for controlling flow of fresh air into the common supplyair duct means and recycling of return air from the common return airduct means to the common supply air duct means, the improvementcomprising partition means for dividing the common return air duct meanslengthwise into a main return air duct section and an auxiliary returnair duct section, separate branch damper means for each zone forcontrolling communication of the branch return duct means of such zonewith each of the return air duct sections, standby means for effectingisolation and exhaust of all return air from the auxiliary return airduct section, and control means including separate detection means foreach zone operable upon the occurrence of fire conditions therein foractuating the standby means and the corresponding one of said separatedamper means for directing all of the return air from any fire zone toexhaust through the auxiliary return air duct section and for directingall of the return air from any non-fire zone into the main return airduct section.

2. An arrangement as defined in claim 1 wherein said standby meansincludes standby exhaust means having a standby damper controllingcommunication of the auxiliary return air duct section to exterioratmosphere, and auxiliary damper means for isolating the auxiliaryreturn air duct section from the main supply air duct means and the mainreturn air duct section to direct the return air in the auxiliary returnair duct section to the standby exhaust means in the fire mode.

3. An arrangement as defined in claim 2 wherein the control means isautomatically responsive to any of the separate detection means upon theoccurrence of fire conditions in the corresponding zone for operatingthe auxiliary damper means to close and isolate the auxiliary return airduct section from the central exhaust- /return fan means for actuatingthe standby exhaust means and standby damper means, for positioning .thebranch damper means for any fire zone so that all return air from anyfire zone is directed through the auxiliary return air duct sectionuntil the corresponding fire detection means returns to normal, forpositioning the branch damper means for any non-fire zone so that allreturn air for any non-fire zone is directed through the main return airduct section when the corresponding fire detection means are normal andfor returning all equipment to normal mode when all fire detection meansreturn to normal.

4. An arrangement as defined in claim 1 where said standby meansincludes variable volume standby exhaust means to operate at a capacitythat depressurizes any fire zones and having a standby damper normallypreventing communication of the auxiliary return air duct section withexterior atmosphere, and auxiliary damper means for isolating theauxiliary return air duct section from the main supply air duct meansand the main return air duct section to direct the return air in theauxiliary return air duct section to the standby exhaust means in thefire mode.

5. An arrangement as defined in claim 4 and including static pressurecontroller means responsive to pressure in the ventilation system tocontrol the capacity of the variable volume stand-by exhaust means toeffect the desired pressure reduction in the fire zones.

6. An arrangement as defined in claim 1 wherein said control means isconnected to actuate said main damper means to control delivery of asmuch as percent fresh air to the ventilation zones during fire mode.

7. An arrangement as defined in claim 1 wherein said common supply airduct means includes air reheat means and said control means is connectedto actuate said main damper means and said air reheat means to controldelivery of as much as 100 percent fresh air to the ventilation zonesduring fire mode regardless of exterior temperature.

8. An arrangement as defined in claim 1 and including variable volumesprinkler means disposed in the auxiliary return air duct section andhaving a sprinkler head and temperature detector for each zone andoperable for automatically controlling return air temperature from anyfire zone.

9. An arrangement as defined in claim 1 wherein each of said separatedamper means comprises a dual damper having a normal mode position todirect return air from the corresponding zone into both return air ductsections and having a fire mode position to direct all return air intothe auxiliary return air duct section when the corresponding zone is afire zone.

10. An arrangement as defined in claim 1 wherein the ventilation zonescomprise separate floors of a multistory building, the common supply airand return air duct means and the partition means are vertical and thebranch supply and return duct means are horizontal and the centralsupply fan means, the central exhaust- /return fan means and the maindamper means are located at the top of the common supply air and returnair duct means and wherein said standby means includes standby exhaustmeans to operate at a capacity that depressurizes any fire zones andhaving a connection to the auxiliary return air duct section at alocation beneath the central exhaust/return fan means and having astandby damper for controlling communication of the auxiliary return airduct section to exterior atmosphere, and auxiliary damper means mountedin the auxiliary return air duct section between the said connection andthe central exhaust/return fan means for isolating the return air ductsection from the main supply air duct means and the main return air ductsection to direct the return air in the auxiliary return air ductsection to the standby exhaust means in the fire mode.

11. A method of fire control and ventilation for multi-zone buildingscomprising: drawing fresh air into the building to form a main supplyair stream; subdividing the main stream into branch streams each leadinginto a separate zone; drawing return air from the zones to form a mainreturn air stream and an auxiliary return air stream; normallyexhausting some of the air from the return air streams while recyclingthe remainder into the main supply air stream; and isolating the returnair streams in response to fire conditions occurring in any zone byexhausting all of the auxiliary return air stream while directing allreturn air from any fire zone into the auxiliary return air stream anddirecting all return air from any non-fire zone into the main return airstream 12. A method in accordance with claim 11 wherein exhaust of allof the auxiliary return air stream in response to fire conditions iseffected at a rate to produce a multifold increase in return air fromany fire zone.

fire zone.

1. In a multi-zone building having a ventilation system that includes acentral station serving a plurality of ventilation zones, common supplyair duct means connecting the central station to the ventilation zones,common return air duct means connecting the ventilation zones to thecentral station, each zone having branch supply duct means connected tothe common supply air duct means and branch return duct means connectedto the common return air duct means, and the central station includingcentral supply fan means, central exhaust/return fan means and maindamper means for controlling flow of fresh air into the common supplyair duct means and recycling of return air from the common return airduct means to the common supply air duct means, the improvementcomprising partition means for dividing the common return air duct meanslengthwise into a main return air duct section and an auxiliary returnair duct section, separate branch damper means for each zone forcontrolling communication of the branch return duct means of such zonewith each of the return air duct sections, standby means for effectingisolation and exhaust of all return air from the auxiliary return airduct section, and control means including separate detection means foreach zone operable upon the occurrence of fire conditions therein foractuating the standby means and the corresponding one of said separatedamper means for directing all of the return air from any fire zone toexhaust through the auxiliary return air duct section and for directingall of the return air from any non-fire zone into the main return airduct section.
 2. An arrangement as defined in claim 1 wherein saidstandby means includes standby exhaust means having a standby dampercontrolling communication of the auxiliary return air duct section toexterior atmosphere, and auxiliary damper means for isolating theauxiliary return air duct section from the main supply air duct meansand the main return air duct section to direct the return air in theauxiliary return air duct section to the standby exhaust means in thefire mode.
 3. An arrangement as defined in claim 2 wherein the controlmeans is automatically responsive to any of the separate detection meansupon the occurrence of fire conditions in the corresponding zone foroperating the auxiliary damper means to close and isolate the auxiliaryreturn air duct section from the central exhaust/return fan means foractuating the standby exhaust means and standby damper means, forpositioning the branch damper means for any fire zone so that all returnair from any fire zone is directed through the auxiliary return air ductsection until the corresponding fire detection means returns to normal,for positioning the branch damper means for any non-fire zone so thatall return air for any non-fire zone is directed through the main returnair duct section when the corresponding fire detection means are normaland for returning all equipment to normal mode when all fire detectionmeans return to normal.
 4. An arrangement as defined in claim 1 wheresaid standby means includes variable volume standby exhaust means tooperate at a capacity that depressurizes any fire zones and having astandby damper normally preventing communication of the auxiliary returnair duct section with exterior atmosphere, and auxiliary damper meansfor isolating the auxiliary return air duct section from the main supplyair duct means and the main return air duct section to direct the returnair in the auxiliary return air duct section to the standby exhaustmeans in the fire mode.
 5. An arrangement as defined in claim 4 andincluding static pressure controller means responsive to pressure in theventilation system to control the capacity of the variable volumestand-by exhaust means to effect the desired pressure reduction in thefire zones.
 6. An arrangement as defined in claim 1 wherein said controlmeans is connected to actuate said main damper means to control deliveryof as much as 100 percent fresh air to the ventilation zones during firemode.
 7. An arrangement as defined in claim 1 wherein said common supplyair duct means includes air reheat means and said control means isconnected to actuate said main damper means and said air reheat means tocontrol delivery of as much as 100 percent fresh air to the ventilationzones during fire mode regardless of exterior temperature.
 8. Anarrangement as defined in claim 1 and including variable volumesprinkler means disposed in the auxiliary return air duct section andhaving a sprinkler head and temperature detector for each zone andoperable for automatically controlling return air temperature from anyfire zone.
 9. An arrangement as defined in claim 1 wherein each of saidseparate damper means comprises a dual damper having a normal modeposition to direct return air from the corresponding zone into bothreturn air duct sections and having a fire mode position to direct allreturn air into the auxiliary return air duct section when thecorresponding zone is a fire zone.
 10. An arrangement as defined inclaim 1 wherein the ventilation zones comprise separate floors of amulti-story building, the common supply air and return air duct meansand the partition means are vertical and the branch supply and returnduct means are horizontal and the central supply fan means, the centralexhaust/return fan means and the main damper means are located at thetop of the common supply air and return air duct means and wherein saidstandby means includes standby exhaust means to operate at a capacitythat depressurizes any fire zones and having a connection to theauxiliary return air duct section at a location beneath the centralexhaust/return fan means and having a standby damper for controllingcommunication of the auxiliary return air duct section to exterioratmosphere, and auxiliary damper means mounted in the auxiliary returnair duct section between the said connection and the centralexhaust/return fan means for isolating the return air duct section fromthe main supply air duct means and the main return air duct section todirect the return air in the auxiliary return air duct section to thestandby exhaust means in the fire mode.
 11. A method of fire control andventilation for multi-zone buildings comprising: drawing fresh air intothe building to form a main supply air stream; subdividing the mainstream into branch streams each leading into a separate zone; drawingreturn air from the zones to form a main return air stream and anauxiliary return air stream; normally exhausting some of the air fromthe return air streams while recycling the remainder into the mainsupply air stream; and isolating the return air streams in response tofire conditions occurring in any zone by exhausting all of the auxiliaryreturn air stream while directing all return air from any fire zone intothe auxiliary return air stream and directing all return air from anynon-fire zone into the main return air stream.
 12. A method inaccordance with claim 11 wherein exhaust of all of tHe auxiliary returnair stream in response to fire conditions is effected at a rate toproduce a multifold increase in return air from any fire zone.
 13. Amethod in accordance with claim 11 and including the step of sensing thepressure in the ventilation system during fire conditions andcontrolling the exhaust of the auxiliary return air stream in accordancewith the pressure in the system to effect a pressure reduction in anyfire zone relative to any non-fire zone.
 14. A method in accordance withclaim 11 and including supplying 100 percent fresh air to all zonesduring the existence of fire conditions while effecting a pressurereduction in any fire zone relative to any non-fire zone.